Surface enhancing coating for gypsum-containing floor underlayments

ABSTRACT

A coating for preparing a floor underlayment to receive an adhesive, where the coating includes a water-soluble, latex polymer, a defoamer and a dual-function surface enhancing agent. The surface enhancing agent includes a set accelerator that also functions to cross link the adhesive, hardening the surface of the underlayment using at least two mechanisms. One embodiment of the invention is a dry mixture that forms a coating when added to water. The dry mixture includes from about 10% to about 50% of the redispersible polymer on a dry weight basis, from about 50% to about 90% of the surface enhancing agent and from about 0.15% to about 0.25% of the defoamer on a dry weight basis. Other embodiments describe a liquid coating formed by adding from about 1 to about 8 parts water to 1 part of the dry mixture on a volume basis.

BACKGROUND

This invention relates to an improved coating for a gypsum-based floorunderlayment and a method of preparing it. More specifically, thecoating provides a harder surface for the floor underlayment andpromotes better adhesion of the coating to the underlayment surface.

Gypsum-based building materials are useful in the building industry in anumber of applications. The materials are prepared by mixing a slurrymade of calcium sulfate hemihydrate (also known as calcined gypsum,plaster of paris or calcium sulfate semihydrate) in water and pouring itinto a mold, forming a panel or floor underlayment, or making the slurryinto any appropriate shape. Water reacts with the hemihydrate to form aninterlocking matrix of calcium sulfate dihydrate (also known as gypsum)crystals, causing the slurry to set and harden. Ideally, when anaggregate is present in the slurry, it is evenly dispersed through thematrix, adding strength and hardness to the resulting matrix layer.Excess water is allowed to evaporate or is driven off by drying thegypsum product. Gypsum is less expensive, lighter weight and fastersetting than concrete. The sound reduction and fire retarding propertiesof gypsum materials are superior to those of wood or other cellulosicbuilding materials.

When used as a floor underlayment, gypsum-containing compositions arecoated with a sealant. A mastic or adhesive is then applied to thesealant, and flooring is applied to the mastic. The flooring materialsinclude vinyl, linoleum, and other floor coverings that are glueddirectly to the underlayment.

However, gypsum-based floor underlayments are not universally acceptabledue to soft surface characteristics and the presence of powder or dustat or near the surface of the underlayment. Under some circumstances,the dihydrate matrix layer with the aggregate substantially dispersedtherein, is topped by a powdery layer that forms on top of the matrixlayer. At the surface, the dust is unsightly and inconvenient to remove.When located just below a surface crust, small amounts of wear cause thesurface crust to break, exposing the powdery layer. Even if time andexpense is invested in removal of the powdery layer, the resultingunderlayment has an uneven surface.

Formation of the powdery layer has been observed when extreme retarders,such as polycarboxylates, are used in the underlayment. Set retardersare frequently used in the slurry composition to lengthen the time thatthe slurry is workable, known as the open time. Increasing the open timeprovides the installers additional time to pour the floor, level it andscreed it before the extent of the setting reactions prevent working ofthe surface. In this case, analysis shows the powdery layer is primarilya mixture of unreacted calcium sulfate hemihydrate and calcium sulfatedihydrate.

Polycarboxylates are known to set retarders but also improve theflowability of the calcium sulfate hemihydrate slurry, providing apourable and/or self-leveling composition using less water. Decreasingthe amount of water in the slurry increases the strength of the productand reduces drying time. The benefits of using polycarboxylates inflooring underlayments are disclosed in co-pending Patent CooperationTreaty PCT/US03/09400, filed Mar. 27, 2003, designating the UnitedStates of America and entitled, “High Strength Flooring Compositions,”herein incorporated by reference in its entirety.

However, the problem of unreacted calcined gypsum is not limited tocases when retarders are used. Environmental conditions, such as wind,result in increased surface drying. Overwatering of the slurry allowsseparation of the aggregate and the gypsum matrix before the settingreactions are complete. Yet another example of a situation that resultsin unreacted calcium sulfate hemihydrate is where there is a change inraw materials. If the aggregate is changed from a wet sand to a drysand, the effective amount of water in the slurry is reduced and in somecases is insufficient to react all of the hemihydrate. Thus, the problemof finding unreacted calcium sulfate hemihydrate at the surface of agypsum-based floor underlayment is not uncommon and can occur in avariety of situations.

Conventional methods of treating underlayment surfaces fail to cure theproblem of unreacted hemihydrate. Merely spraying water on the powderylayer is not sufficient to hydrate a significant portion of theunreacted hemihydrate. Application of conventional topcoating materials,such as TOPSEAL (United States Gypsum Company, Chicago, Ill.),temporarily seals the dust between the coating and an interlockingmatrix of calcium dihydrate crystals, but does not provide a long-termsolution to this problem. When the powdery layer forms under the surfacecrust, the coating layer adheres to the crust as it dries, but thecoating and attached crust easily peal away from the underlying matrixlayer because the powdery layer does not attach the crust to the strongmatrix layer. In places where the powdery layer is thin, the coating canpenetrate through the powdery layer to bond to the dihydrate matrixstructure in isolated places. However, isolated bonds may not be strongenough to hold the coating in place over the entire surface. As thecoating over the unbonded areas pulls and shifts under normal use, thebonded areas are pushed and pulled in many directions by movement of thecoating. Changing pressures can cause the small patches of bonded gypsumto break up, eventually causing delamination of all or of large portionsof the coating.

Another method of combating this problem would be through the use ofadditives to the calcined gypsum slurry. However, this techniquerequires that one accurately guess in advance the amount of the powderylayer that will appear as the underlayment sets. Variables such aswetness of the aggregate, wind conditions, temperature, type and amountsof set retarders used would have to be considered, resulting in acomplex algorithm for determining if such additives would be necessary.If the amount of unreacted calcium sulfate dihydrate differed from thepredicted amount, or if conditions changed in the middle of the job, theamount of the additive would have to be reconsidered, requiringreevaluating all of the above variables. This could lead to a lengthyand complex process that installers will not want to use.

Thus, there is a need in the art for a coating to strengthen the surfaceof a floor underlayment, especially when the underlayment has a powderylayer at or near its surface. The coating should also react withunreacted calcium sulfate hemihydrate to improve the strength of thepowdery layer. The coating should be easy to apply using commonlyavailable tools or methods.

SUMMARY OF THE INVENTION

These and other benefits are found in the coating of the presentinvention. The coating promotes hydration of the unreacted calciumsulfate hemihydrate to form an interlocking matrix of calcium sulfatedihydrate, as well providing a strong, stable surface for adhesion ofmastic or adhesives for subsequent floor coverings such as vinyl orlinoleum that are installed on top of the coating.

More specifically, this coating includes a film-forming polymer, adefoamer and a dual-function surface enhancing agent. The surfaceenhancing agent includes a salt that acts as a gypsum set acceleratorbut also functions to cross link the mastic that is subsequentlyapplied, enhancing the surface of the underlayment by at least twomechanisms. One embodiment of the invention is a dry mixture that formsa coating when added to water. The dry mixture includes from about 10%to about 50% of the redispersible polymer on a dry weight basis, fromabout 50% to about 90% of the surface enhancing agent and from about0.15% to about 0.25% of the defoamer on a dry weight basis. Otherembodiments describe a liquid coating formed by adding from about 1 toabout 8 parts water to 1 part of the dry mixture on a volume basis.

A coated floor underlayment is formed by coating an uncoated floorunderlayment containing gypsum with the coating including water, thefilm-forming latex polymer, the surface enhancing agent and thedefoamer. After the coating has been applied and dried, a multi-layerfloor underlayment is formed that includes a matrix layer that includesaggregate particles dispersed in a crystalline matrix of calcium sulfatedihydrate, and a surface layer that includes calcium sulfatehemihydrate, the polymer and a salt dispersed in a calcium sulfatedihydrate matrix.

A method for strengthening the surface of a gypsum-based floorunderlayment includes providing a set and dried floor underlayment.After selecting a redispersible polymer and surface enhancing agent, thepolymer and the surface enhancing agent are blended with a defoamer toform a mixture. At the time of application, the mixture is combined withwater at a ratio of from about 1:1 to about 1:8 by volume, then mixed toform a liquid coating. The coating is then applied to the surface of thefloor underlayment and allowed to dry.

Because the coating of the present invention is applied to the setgypsum-based underlayment, there is no need to guess whether or not thepowdery layer will form or if so, how much unreacted calcium hydratewill be present. Added chemicals and expense need not be added to theslurry if the powdery layer does not form. Only after the underlaymentis substantially set, and it is known whether or not the powdery layerforms, is it necessary to make a decision as to whether or not to treatthe powdery layer. If no powdery layer forms, a more conventionalcoating is usable to seal the underlayment, or no coating at all need beused on the floor. When it is necessary to treat the powdery layer, moreinformation is available to decide on the exact coating composition orwhether it would be advantageous to apply multiple coating layers.

The dual-functionality of the coating treatment results in a surfacethat is significantly harder and has better adhesion between the solidlayer of aggregate-containing calcium sulfate dihydrate matrix and themastic. Because the coating is more firmly anchored to the strongaggregate layer, it provides a more stable surface to which the masticfor vinyl or other flooring products to adhere. Action of the surfacehardener works in two ways. First, it promotes hydration of theunreacted calcium sulfate hemihydrate, building a more extensive matrixof interlocking crystals of calcium sulfate dihydrate within the powderylayer. This matrix adds considerable strength to the powdery layer withthis three-dimensional structure, and increases occurrences where thedihydrate matrix spans the entire powdery layer to tie in to thedihydrate matrix of the aggregate layer to the coating film.

Secondly, presence of the metal salts improves bonding of the mastic tounderlayment surface. Surface analysis suggests that metal salts form ina surface layer. It is theorized that when the mastic is applied thesesurface salts promote cross-linking of the mastic and interfacialbonding between the mastic and the newly forming gypsum matrix.Cross-linking of the mastic makes it harder, while increasing the numberof chemical bonds between the mastic and the gypsum matrix improves theadhesion at the interface. Regardless of the mechanism, bonding of themastic to the underlayment is improved.

The coating of the present invention is easy for contractors to use. Itis available in a dry powder form that has a long shelf life and islightweight to carry to a job site. It is mixed with water on site on avolume basis. The coating is applied using standard coating techniques,without any special equipment. No complex process is needed to determinethe amount of coating to be used and the coating provides an improvedsurface even if the powdery layer does not form on the surface of theunderlayment.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows three samples of vinyl flooring on Test Panel 1 preparedaccording to Examples 1-3;

FIG. 2 shows Test Panel 1 and the results of the peel-off test ofExamples 1-3;

FIG. 3 shows Test Panel 2 with results of the peel-off test of Examples4 and 5;

FIG. 4 shows Test Panel 3 and the results of the peel-off test ofExample 6;

FIG. 5 shows Test Panel 4 and the results of the peel-off test ofExample 7; and

FIG. 6 shows Test Panel 5 and the results of the peel-off test ofExample 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a coating or coating mixture for afloor underlayment and a floor treated with the subject coating. Thecoating or coating mixture for a floor underlayment of the presentinvention includes a film-forming latex polymer, a surface enhancingagent selected to both accelerate the setting reactions of calcinedgypsum and to form a surface layer that bonds strongly with mastic; anda defoamer. Although water is necessary for application of the coatingto the floor underlayment, a coating mix is advantageously supplied inthe form of a dry powder with instructions that water be added justprior to application of the coating to the underlayment. In anotherembodiment, the coating is ready-mixed with water and is supplied readyfor application. It is to be understood in the following discussionthat, although the coating mixture is not always mentioned, the coatingis readily obtainable by combining the coating mixture with water. It isfurther contemplated that the choice of raw materials is not limited todry powder forms when a liquid coating or concentrate is prepareddirectly.

One component of the coating is the redispersible latex polymer. Thepolymer forms throughout the new calcium sulfate dihydrate matrix,providing a hard and stable surface to which the mastic adheres, bindingsubsequent flooring layers to the underlayment. Any redispersible latexpolymer is contemplated for use in this coating that is compatible withthe surface enhancing agent and that enhances the interaction betweenthe surface enhancing agent and the mastic. Examples of useful latexpolymers include polyacrylates, polymethacrylates, styrene-butadienecopolymers, vinyl acetate co-polymers, vinyl chloride co-polymers,polybutadienes, and polyisoprenes. Polyacrylic resins and vinylacetatecopolymers are preferred resins. Especially preferred resins arevinylacetate ethylene copolymer and vinylacetate-vinylveresatatecopolymers, particularly ELOTEX Flowkit 51 (National Starch Co.,Chicago, Ill.). Use of any particular structures, includinghomopolymers, copolymers, terpolymers and the like, is contemplated.

The polymer is compatible with the surface enhancing agent if it doesnot react with the surface hardening agent in the liquid phase. If thepolymer and the surface hardening agent react to form a precipitate orinsoluble species, then the original chemical species will not bepresent when the coating is applied to the floor underlayment. Anyprecipitate or new species formed would further alter the reactions thattake place when the coating is applied. The chemical reactions shouldbegin when the coating contacts the powdery layer. Hydration of thecalcium sulfate hemihydrate absorbs water and water is lost toevaporation at the surface. As the amount of available water decreases,the latex begins to coalesce and salts formed by the surface enhancingagent begin to salt out. Progress of all of these events simultaneouslycreates a matrix of interlocking calcium sulfate dihydrate crystals,having the latex polymer, precipitated salts and unreacted calciumsulfate hemihydrate dispersed through the matrix.

Another consideration in selecting the polymer is its ability to enhancethe interaction between the surface enhancing agent and the mastic orflooring surface that is subsequently applied. If the polymer forms acontinuous film between the underlayment and the mastic or flooringsurface, the surface enhancing agent will not contact the mastic topromote cross-linking, as discussed below. The type and amount of latexpolymer are selected so that at least some discontinuities exist in thepolymer film to expose crystals of the surface hardening agent,enhancing contact and interaction between the layers.

Any effective amount of the polymer is useful in this coating. Aneffective amount is one that does not react with the surface enhancingagent in the liquid phase and enhances interaction between the surfaceenhancing agent and the mastic or flooring. Preferably the amount ofpolymer is from about 10% to about 40% based on the weight of drysolids. Polymer concentrations of about 15% to about 25% are morepreferred.

The surface enhancing agent is used to strengthen the surface of theunderlayment, prevent peeling of the coating after it is dry and promotebonding with the mastic that is subsequently applied. It functions toaccomplish these goals in two ways. First, it acts as a set acceleratorto hydrate the unreacted calcium sulfate hemihydrate in the powderylayer. The calcium sulfate hemihydrate that remains unreacted after theunderlayment has set resists hydration so that application of plainwater to the powdery layer results in negligible additional hydration.When recommended amounts of the surface enhancing agent are used, about30% of the calcium sulfate hemihydrate reacts further to form calciumsulfate dihydrate. This extent of reaction forms a sufficient member ofcalcium sulfate dihydrate crystals to bond the coating to the underlyingmatrix layer.

The second function of the surface enhancing agent is enhanced bondingof the coating of the mastic. A gypsum set accelerator that does notalso enhance bonding with the mastic is ineffective as the surfaceenhancing agent. Cross-linking of the mastic attaches it to the formingdihydrate matrix and increases its molecular weight, making it harderand more resistant to delamination. Free radicals or metal ions areknown for use as cross-linkers.

The concentration of the surface enhancing agent in the coating mix ispreferably from about 50% to about 90% on a dry weight basis. Morepreferably, the concentration is from about 75% to about 85%. When lowconcentrations of surface enhancing agent are used, the amount ofcoating applied is increasable, if desired, to maintain a relativelyconstant conversion of calcium sulfate hemihydrate.

Any substance that performs both of these functions is usable as thesurface enhancing agent. Compounds that have been found to be mostuseful are ionic in nature. Metallic cations are used to promotecross-linking of the mastic. The cation is preferably multivalent,especially a divalent or trivalent ion. Divalent or trivalent metalions, such as zinc, magnesium, copper, aluminum, ferrous or ferric ionsare particularly preferred. Monovalent cations, such as potassium orsodium appear to have lower activity, but are useful when the surfaceenhancing agent is present in the higher concentrations in the liquidcoating.

Any anions that are known as set accelerators, including sulfates,nitrates and chlorides, are contemplated for use in the coating.Preferred salts include, but are not limited to, potassium sulfate,sodium sulfate, magnesium sulfate, copper sulfate, ferrous sulfate,ferric sulfate and aluminum sulfate. Salts having multiple cations,including sodium hydrogen sulfate or potassium hydrogen sulfate, areexpected to be useful as well. Zinc sulfate is a particularly preferredsurface enhancing agent, but use of any material that serves the dualfunctions described above is contemplated for use in this coating.Oldbridge Zinc (Harcos Chemicals, Bellesville, N.J.) or SUPER TEL Zinc(Tetra Micronutrients, Fairbury, Nebr.), are preferred sources of highpurity grade zinc sulfate.

The defoamer is used to reduce the foaming that occurs when the drycomponents are mixed with water. Any defoamer that is readily availableand does not adversely interact with other components is usable. AGITANP 823 (Ultra Additives, Inc. Paterson, N.J.) is a preferred defoamer andis available in a powdered form. The defoamer is most useful in thecoating in concentrations of about 0.15-0.25% based on dry weight.Preferably, about 0.19 to about 0.21% defoamer is used on a dry weightbasis.

Water is used as a solvent for the coating and to hydrate the calciumsulfate hemihydrate. A sufficient amount of water is one that hydratesthe reacting calcium sulfate hemihydrate, disperses the polymer,dissolves the surface enhancing agent and forms an emulsion having theflowability to coat the underlayment and allow the polymer to coalesceinto a film. Use of additional water over the sufficient amount thinsthe coating and makes it more flowable, but also adds to the dryingtime. The maximum amount of water is controllable by the time availableto allow the coating to dry in anticipation of applying an adhesive oradjacent flooring layer. When a coating of a certain thickness isdesired, more total coating is required when the coating is dilutedcompared to a coating using a lesser amount of water.

In a preferred embodiment, the polymer, defoamer and surface enhancingagent are supplied premixed in dry form. At the job site, the water isadded to the dry coating mix to prepare the coating. Preferably, thevolume of water added is at least equal to the volume of the dry coatingmix. In some embodiments, the ratio of the volume of coating mix towater is from about 1:1 to about 1:8. More preferably, water is used ina volume ratio of from about 1:3 to about 1:5 or at a ratio of about1:4. The same ratios of water to dry solids are useful when the water isadded at the time of manufacture to a ready-mix formulation. Manufactureof a ready-mix concentrate is also contemplated, whereby a portion ofthe water is added at the time of manufacture and another portion of thewater is added just prior to application. The concentrate is expected tobe more economical by reducing the cost of shipping and storing largequantities of water.

It is also contemplated that multiple treatments be used to furtherincrease the portion of calcium sulfate hemihydrate in the powdery layerthat is hydrated. If the polymer film does not form a water barrier,application of additional coating penetrates the film to reactadditional calcium sulfate hemihydrate. Treatments are repeatable untila sufficient amount of calcium sulfate hemihydrate is converted, oruntil a sufficient layer of polymer has built up to form a film that isimpenetrable by the coating solution.

After one or more treatments with the surface enhancing coating, thenature of the coating surface is changed. The powdery layer becomes a“surface layer.” When the mastic is applied, this surface layer acts asa transition layer between the matrix layer and the mastic to hold theflooring in place. When the coating is applied to the powdery layer,calcium sulfate hemihydrate begins to absorb water of hydration to forman interlocking matrix of calcium sulfate dihydrate crystals. Uponapplication, water also begins to evaporate from the surface of theunderlayment. As the water evaporates or becomes associated with gypsummolecules, the liquid becomes more concentrated. Disappearance of thewater causes the polymer to coalesce and begin to form and the ions fromwater impurities and the surface enhancing agent begin to salt out.

Salts that form will be composed primarily of the ions of the surfaceenhancing agent. However, other ions present will also form salts as theliquid becomes too concentrated to hold everything in solution. In someareas, the water is likely to have hard water ions, including calciumand magnesium. Other locations have rust as a common water component.Inclusion of salts of these ions with the surface enhancing agent isunlikely to be detrimental, and can actually improve cross-linking ofthe mastic.

As more and more water is absorbed or evaporated, several events aretaking place at once. The salts are precipitating out at the same timethe polymer is coalescing and forming. Substantially simultaneously, thematrix of dihydrate gypsum is developing. As there are no forces actingto separate the components as these events are taking place, the gypsummatrix forms with the polymer, salts and unreacted calcium sulfatehemihydrate dispersed through the matrix. The newly formed dihydratematrix also interlinks with the dihydrate matrix of the matrix layer,bonding the two layers and reducing the tendency for interfacialseparation.

A number of optional components are added to the coating when it isdesirable to impart specific properties to the coating. Any additiveconventionally used with either the underlayment or the coating thatdoes not adversely react with the required components is contemplatedfor use in the surface enhancing coating.

Certain optional additives are preferably used when the coating ismanufactured in a liquid emulsion or ready-mix form. For example, theaddition of an antifreeze is preferred, such as propylene glycol,ethylene glycol or any other material that does not adversely effectother components. Any useful amount of antifreeze is contemplated, butis it preferably used in amounts of from about 0.5% to about 3%. Morepreferably, antifreeze is added in amounts of from about 0.8% to about1% based on dry solids.

The addition of a biocide is also highly preferred in ready-mixformulas. Any biocide is expected to be useful in the present coatingunless it interacts with other components of the mixture. A preferredbiocide is KATHON LX 1.5 (Rohm & Haas, Bristol, Pa.). This biocide isespecially preferred when used in amounts of from about 0.1% to about0.2% based on dry solids.

Use of a wetting agent is optional, but highly preferred. Preferredwetting agents include nonyl phenol, Igepal CO-660 and CO-630 (GAFCorporation, Wayne, N.J.) and Tritron X-100 wetting agent (Rohm & Haas,Bristol, Pa.). Suitable wetting agents are also made by Rhone-Polonc andother makers of wetting agents and detergents. The wetting agent ispreferably used in amounts of less than 1% by weight of the dry solids.More preferably, the wetting agent is present in amounts of about 0.2%to about 1%, and even more preferably about 0.5% based on the dry solidsbasis.

Strength enhancers are optionally added to strengthen or reinforce thematrix of calcium sulfate dihydrate that forms when the hemihydrateundergoes hydration. Suitable enhancers for dimensional stability andresistance to permanent deformation are disclosed in U.S. Pat. No.6,632,550, herein incorporated by reference in its entirety. Strengthenhancers described therein include condensed phosphoric acids, each ofwhich has 2 or more phosphoric acid units and salts or ions of condensedphosphates, each of which has 2 or more phosphate units. Any additivethat does not adversely effect other components is contemplated for usehere, but sodium trimetaphosphate is the most preferred. Preferably, thestrength enhancer is added in amounts ranging from about 0.004% to about2% based on the dry solids weight, and more preferably in amounts ofless than about 0.1%, or amounts of about 0.08%.

A mildewcide is a recommended additive in either the dry mix or liquidcoating form. Pyrithiones, such as sodium OMADINE or zinc OMADINE (ArchChemicals, Norwalk, Conn.), are preferred mildewcides, although it iscontemplated that any mildewcide that does not adversely interact withother components is usable in this coating. Preferably, the mildewcideis present in amounts ranging from about 0.03% to about 0.05% based ondry solids.

To assist in determining whether an underlayment has been treated orthat treatment was complete, a visual indicator is optionally added toallow for visual confirmation that the coating has been applied.Pigments, optical brighteners and pH indicators are preferred, but anyother type of visual indicator is usable.

A visible color change is imparted to the coating when pigments areadded, and that color change is visible on the underlayment to which thecoating has been applied. Pigments or dyes are also useful for aestheticpurposes. Typically they take the form of powders, flakes, organic ororganometallic molecules and the like. Examples of suitable pigments ordyes include iron oxide, other metal oxides, titanium dioxide and carbonblack.

Optical brighteners are fluorescent white dyes, commonly present inlaundry detergents, that absorb ultraviolet “UV” light and fluoresce inthe blue region of the visible spectrum. When added to a coating, noobvious color change occurs, but the fluorescence is detectable in thepresence of UV light. Thus, shining an ultraviolet light over anunderlayment allows the owner or installer to verify that theunderlayment has been treated and that coverage was complete, withoutimparting a particular color to the underlayment. Optical brighteners,such as TINOPAL SFP (Ciba Specialty Chemicals, Tarrytown, N.Y.) arepreferred visual indicators.

Indicators that temporarily change color due to changes in pH are alsosuitable for determining where the coating has been applied. Forexample, applying ammonia other basic solution over a coating thatincludes phenolphthalein would cause the coating to change fromcolorless to red. Any pH indicator that does not adversely interact withother components of the coating is usable in this embodiment, includingthose that change color in the presence of acid.

Appropriate amounts of the indicator depend on the type of indicatorselected. Any amount of indicator is useful that produces a noticeablecolor change but does not adversely effect other components of thecoating composition. Preferably, the visual indicator is added inamounts of from about 0.03% to about 2% based on dry solids. Whenpigments are used, amounts at the higher end of the range are expectedto be most satisfactory. Only small amounts of optical brighteners areneeded, and are usable in amounts near the lower end of the preferredrange.

In some embodiments of the invention, the coating is applied to asuitable gypsum-containing floor underlayment. The term “underlayment”is intended to refer to any material that raises or supports flooringthat is attached or mounted to it. Preferably the underlayment includesgypsum, cement or mixtures thereof, but any underlayment having arelatively soft surface, including wood, clay, and the like, is likelyto benefit from application of this coating. Gypsum is preferablypresent in the underlayment in amounts from about 50% to about 95%. Theunderlayment is typically supplied in panels or a gypsum-based slurry ispoured directly into the space to be occupied by the underlayment andallowed to set in place. However, the form by which the underlayment issupplied is unimportant and any form is usable with the coating of thisinvention.

A preferred underlayment for use with this coating is made from amixture to be used in conjunction with water for preparing a slurry thathydrates to form a high strength flooring compound. The mixture includesfrom about 30% to about 98% by weight calcium sulfate hemihydrate,having at least 25% of the calcium sulfate hemihydrate in either thealpha-calcined form, the beta-calcined form or a combination thereof. Apolycarboxylate dispersant is included in the mixture in amounts fromabout 0.2% to about 10% by weight. The mixture also includes 0.05-50% byweight of either cement or lime. When combined with water in amounts ofabout 10% to about 50% water based on the weight of the dry flooringmixture, a slurry is formed that is useful as a flooring composition.This underlayment composition is disclosed in Patent Cooperation TreatyPCT/US03/09400, previously incorporated by reference.

Any mastic or adhesive is useful with this coating that includes atleast one component cross-linkable by the surface enhancing agent.Optimum surface hardness is obtained when conversion of unreactedhemihydrate to dihydrate and cross-linking of the mastic both occur. Anycomponent of the adhesive that is available to react with the coatingsurface to cross-link the adhesive is useful in this invention.

Flooring adhesives used for vinyl sheet goods installations usuallyinclude a tackifying resin and solids, with optional inorganic fillers.The tackifying resin, which is usually proprietary, increases initialadherence of the adhesive to the flooring. The solids are preferably anintimately blended mixture of high molecular weight latex polymerscharacterized by having at least 50 percent by weight solids and aninorganic filler. Adhesives containing acrylic type latex polymers arepreferred because the polymer reacts with metal ions such as zinc,copper, and aluminum to cross-link the mastic through the formation ofmetal-acrylate complexes.

In other preferred adhesives, a cross-linkable stabilizing colloid isoptionally added for in-can and mechanical stability. In the case the ofstyrene-butadiene rubber (SBR) type adhesive, the stabilizing colloidwill also react with zinc, copper, and aluminum ions throughmetal-carboxylate complexes.

Thus, the floor underlayment treated with the coating of this invention,having a metal ions such as zinc on the surface layer, will react withthese adhesives to cross-link one of more components of the adhesivethrough the formation of metal-polymer complexes. Commercially availableadhesives such as Congoleum 3044 (Congoleum, Mercerville, N.J.),Mannington V-31 (Mannington Mills, Salem, N.J.), Armstrong S-254(Armstrong World Industries, Inc., Lancaster, Pa.), and Tarket FB-1(Tarket Floors, Johnson City, Tenn.), are useful with the coating ofthis invention for enhancing the adhesion between adhesive and floorunderlayment.

The coating mix is made by combining dry ingredients and blending them.In a preferred embodiment, a dry coating mix is prepared by mixing adry, redispersible polymer, a powder defoamer and a dry surfaceenhancing agent. These components are preferably blended until asubstantially uniform dry mixture is obtained. Optional components arealso added and blended into the mixture. The mixture is then optionallypackaged and sold in the dry form.

At the job site, the coating is prepared by mixing the dry coatingmixture with water. Preferably the dry powder is slowly added into anappropriate quantity of moving water and mixed for at least one or twominutes. The solution is preferably allowed to rest for about 15minutes, then remixed for an additional 1-2 minutes. The mixing methodis not important, stirring with a paddle by hand or mixing with amechanical device are equally effective in producing a high qualitycoating.

Liquid ready-mix formulations have the water added prior to packagingand distribution. They are generally more expensive than a dry coatingmix because the water is packaged, shipped and stored with the othercomponents. However, in some situations, such as when very smallquantities are used, or when suitable water is not available at the jobsite, paying a premium for a ready-to-use coating is justifiable. Thesame technique for mixing the ingredients is used as for the dry coatingmixture, except that the packaging and distribution of the product takesplace after the coating is finished and ready-to-use. Preparation of aready-mix concentrate is also contemplated that is a liquid form, buthas less water than the ready-mix product. The concentrate isreconstitutable in a manner similar to the dry mixture, by adding theconcentrate to a sufficient amount of water for optimum dilution andmixing thoroughly. Whether the coating is prepared from a dry mix at thejob site, or prepared at a plant and sold in a ready-mix form, it ispreferable to stir the coating just prior to application to obtain asubstantially uniform consistency.

The coating is applied to the set and dried underlayment by any knowncoating method. Preferred methods of application include spraying,rolling, mopping, squeeging, backrolling, pouring, trowelling, brushing,or otherwise coating the surface of the substrate. Combinations of thesetechniques are also useful, such as spraying the coating to distributeit then brushing the coating to obtain a more uniform finish.

Thickness of the coating often depends on the particular substrate, thecoating composition and the degree of surface hardness desired.Preferably the coating is applied to achieve a coating weight of 10-14grams coating per square foot. Thicker or thinner coatings are alsouseful under particular circumstances. For example, thicker applicationsof the coating are beneficial where the substrate is rough or uneven,where the powdery layer is particularly thick, or if the coatingsolution is dilute.

Following application, the coating is allowed to dry prior toapplication of additional flooring layers such as adhesives, mastic orvinyl flooring. Upon drying, the coating composition coalesces to form afilm. Preferably, the coating is allowed to dry at least 2 hours, morepreferably at least 4 hours, to assure adequate drying. Longer orshorter drying times are suitable depending on the coating thickness,the ambient temperature and humidity, and the amount of water present inthe coating. The maximum drying time is limited only by the timeavailable before the final flooring is laid.

This coating is useful alone or together with other coatings orwaterproofing systems. Conversion of unreacted calcium sulfatehemihydrate occurs only when the coating composition comes in contactwith the coating solution. When the powdery layer is present, it isadvantageous to position the present coating adjacent the powdery layer.Where a water barrier is needed, the use of a conventional sealantbetween the subject coating and the mastic is contemplated.

In the discussion above and in the examples that follow, all rangesexpressed in percentages are based on the dry weight of the componentspresent in the coating. Water addition is measured in terms of volumesof water per volume of dry components. The examples are not intended tobe limiting of the invention, but merely to demonstrate making and useof some of the embodiments described herein.

EXAMPLES 1-3

Test panels of LEVELROCK LR2500 (USG Corporation, Chicago, Ill.) wereprepared for testing of coating preparations. Plywood molds one footsquare and one inch deep (30 cm×30 cm×2.5 cm) were prepared by primingthem with USG Underlayment Primer (USG Corporation, Chicago, Ill.).LEVELROCK LR2500 Underlayment was mixed according to the manufacturer'sdirections and cast into the molds. The samples were allowed to air-dryfor about four weeks, or until they achieved a constant weight. Thesample was divided three equal test areas in one direction, each testarea being approximately 4″×12″ (10 cm×30 cm). Several strips of vinylsheet flooring were cut, each to fit within the test area.

Three comparative examples were prepared testing the components used inthis invention separately. Water alone was tested alone to see if itwould hydrate a sufficient amount of the unreacted hemihydrate to form astable dihydrate matrix. Zinc sulfate and latex polymer, in the form ofTOPSEAL coating (USG Corporation, Chicago, Ill.), were each testedseparately. Compositions of each of these examples is shown in TABLE I:TABLE I Example 1 2 3 Position on Test Left Center Right Panel 1 CoatingType Water Zinc Sulfate TOPSEAL Coating Weight 3 grams 3 grams 3 gramsPolymer 0 0 89.07% dry Wt. Zinc Sulfate 0 100% dry Wt. 0 Water added to100% 67% by Wt. 80% by Vol. dry ingredients

TOPSEAL is a commercially available sealant for floor underlayments. Inaddition to the required components, the commercial formula used inExamples 3 and 4 is a liquid concentrate that also includes a smallamount of water. The complete composition of the USG TOPSEAL formulationis shown in Table II. TABLE II TOPSEAL COMPOSITION Component Source %Wt. Rhoplex 8764L Rohm & Haas Co. 89.1 (Philadelphia, PA) PropyleneGlycol 0.92 Kathon 1.5 Rohm & Haas Co. 0.20 Skane M-8 Rohm & Haas Co.0.16 DeFoo 542 Ultra Additives Co. 0.28 CO-630 Rohm & Haas Co. 1.22Water 8.2

The comparative example of test area 1 was coated with approximatelythree grams of water and allowed to dry. Test area 3 was coated withthree grams of commercially available TOPSEAL coating that was preparedby mixing in a 1:4 ratio with deionized water by volume. TOPSEAL is apolymeric coating having minor amounts of standard additives such asdefoamers, wetting agents, biocides, etc., but no surface enhancingagent. Test area 2 was coated with an aqueous solution of a zincsulfate, a surface hardening agent. For each example, the coatings wereapplied to the surface of the underlayment with a brush. The loading ofthe coating on the underlayment surface was 12 grams per square foot(129 g/m²) or approximately three grams of coating within each testarea. Each of the coatings was allowed to dry thoroughly, at least 2hours, before the test proceeded.

Following application of the coatings, mastic was uniformly applied tothe coatings with a {fraction (1/16)}″ (1.6 mm) square-notched adhesivetrowel held at a 45° angle. Use of the adhesive trowel results in themastic being applied in a series of parallel channels of uniformthickness at a rate of 34.7 grams mastic/ft² (374 g/m²). The samples ofvinyl sheet were applied 15 minutes after the mastic was applied. Aroller 6 inches (15 cm) wide and weighing 53 pounds (24 Kg) was appliedto each of the samples. The roller supplied uniform pressure on allparts of the vinyl to evenly distribute the mastic. All samples wereair-dried for about 24 hours before the peel-off tests. FIG. 1 shows thetest panel after all test areas were prepared.

When all samples were dry, peel-off tests were carried out to determinethe ease with which the samples are removable from the underlayment.Upward pressure is applied by hand trying to separate the vinyl from themastic and the underlayment. The upward force is increased until one ofthe bonds in the underlayment/mastic/vinyl system fails or until thesystem exceeds the strength of the tester. Weakest bonds fail first. Ifthe bond between the underlayment and the mastic fails, the mastic willbe attached to the vinyl. When the adhesion between the mastic and thevinyl is the weakest, the striped channels of the mastic are clearlyvisible and are attached to the underlayment. If failure occurs withinthe layers of the flooring sample, the mastic remains attached to theunderlayment, but a portion of the flooring obscures the stripes of themastic. Results of peel-off tests of Examples 1-3 are shown in FIG. 2.

Test area 1 using water only shows that none of the vinyl and only someof the mastic have adhered to the underlayment. Virtually none of thecalcium sulfate hemihydrate was converted to the dihydrate, allowing thevinyl and mastic to easily separate from the matrix layer at the powderylayer. The zinc sulfate solution of the center sample shows improvedadhesion. Delamination of the vinyl demonstrates good bonding betweenboth the mastic and the underlayment and the mastic and the vinyl. Useof TOPSEAL sealant in Test Area 3 on the right shows some delaminationof the vinyl, but in some areas the vinyl as separated cleanly from themastic.

EXAMPLES 4-5

The coating of this invention, SE, was prepared using the components ofTable II. The first two dry components were placed in a mixer and mixedfor about 1½ minutes. The remaining components were then added to themixture. Upon mixing, the resulting mixture was a white, free-flowingpowder. The dry blend was then added to deionized water at a ratio of 1part dry mixture to 4 parts water based on volume and mixed. TABLE IIIComponent Source Weight, grams Zinc Sulfate 79.6 Flowkit 51 PolymerNational Starch 19.9 Agitan P-823 Ultra Additives, Inc. 0.49 Paterson,NJ Biocide 0.1 Optical Brightener 0.1

A second test panel was prepared as described in Examples 1-3 above anddivided into two test areas of approximately equal size. Samples ofvinyl sheet were cut to fit within each test area. The TOPSEAL coatingof example 3 was applied to the test area on the left of the panel, andis designated Example 4. Example 5 is shown on the right side of thetest panel, where the SE coating was applied. Mastic was applied asdescribed in Examples 1-3 above, and the two vinyl samples were pressedinto the adhesive using the roller. Table VI summarizes the examples ofTest Panel 2. TABLE VI TEST PANEL 2 Example 4 5 Position on Test LeftRight Panel 2 Coating Type TOPSEAL SE Coating Weight 12 g/ft² (129 g/m²)12 g/ft² (129 g/m²)

Results of the peel-off tests are shown in FIG. 3. Example 4 isessentially a duplicate of Example 3. As discussed above, the stripedyellow areas are sections where mastic is visible because there isvirtually no adhesion. The coating of this invention, shown in Example5, shows significant improvement in adhesion. Mastic is visible onlydown the right edge of the sample and a small area in the lower leftcorner. The SE formulation of Example 5 shows strong adhesion anddelamination of the vinyl over virtually the entire sample.

Additional test panels were prepared using the method described above tomake larger panels 4 ft×4 ft×1 in (1.2 m×1.2 m×2.5 cm). In Examples 6and 7 that follow, the SE coating of Example 5 was tested with a varietyof flooring samples to verify that the coating is effective withflooring of different composition and construction. Samples wereprepared in the same manner described in Example 1 with one exception.Due to the larger sample size, a large, 3 Sectional 15 inch (37.5 cm)Floor Roller weighing 103 pounds (46.7 Kg) (Beno Gundlach Tools,Belleville, Ill.) was used to set the vinyl into the mastic. The SEcoating was applied to a coating weight of 12 grams/ft² (129 g/m²).

EXAMPLE 6

In Example 6, Mannington vinyl sheet flooring (Mannington Mills, Inc.Salem, N.J.) was applied to the mastic and allowed to dry for 24 hours.To minimize edge effects, a utility knife was used to remove a piece ofthe flooring, and manual force was applied to the vinyl to try toseparate it from the underlayment. Results are shown in FIG. 4,revealing that the backing has adhered to the mastic and the vinyl hasdelaminated.

EXAMPLE 7

Example 6 was repeated using Congoleum vinyl sheet flooring (CongoleumCorporation, Mercerville, N.J.). Again, no mastic is visible because thevinyl has delaminated, indicating firm bonds of the mastic to the vinyland to the underlayment.

EXAMPLE 8

This example demonstrates the viability of using the SE coating togetherwith a conventional sealant. A test panel was prepared as described inExample 6 above. The underlayment was first coated with 12 grams/ft²(129 g/m²) of the commercial SE formulation of Example 5 and allowed todry for about 4 hours. Next, the underlayment was coated with about 10grams/ft² (107 g/m²) of the commercial TOPSEAL sealant. When the TOPSEALwas completely cured, the mastic and samples of vinyl sheets wereapplied to the surface. FIG. 6 shows the very good adhesion of sheetvinyl even when both coatings are used together. The application ofTOPSEAL sealant on top of the SE coating results in hardening of thegypsum surface, good adhesion and formation of a water barrier betweenthe underlayment and the final flooring.

While a particular embodiment of the present invention has been shownand described, it will be appreciated by those skilled in the art thatchanges and modifications may be made thereto without departing from theinvention in its broader aspects and as set forth in the followingclaims.

1. A mixture for use with water as a coating between a floorunderlayment and an adhesive, said mixture comprising: from about 10% toabout 50% dry weight of a redispersible, latex polymer; from about 50%to about 90% dry weight of a surface enhancing agent, said surfaceenhancing agent being selected to function as both a set accelerator forcalcined gypsum and also as a cross-linking agent for the adhesive; andfrom about 0.15% to about 0.25% dry weight defoamer.
 2. The mixture ofclaim 1 wherein said surface enhancing agent is at least one of adivalent cationic sulfate salt and a trivalent cationic sulfate salt. 3.The mixture of claim 2 wherein said divalent cationic sulfate saltcomprises zinc sulfate.
 4. The mixture of claim 1 further comprising awetting agent.
 5. The mixture of claim 4 wherein said wetting agent ispresent in amounts of from about 0.2% to about 1% by weight based on thedry solids.
 6. The mixture of claim 1 further comprising a mildewcide.7. The mixture of claim 6 wherein said mildewcide is present in amountsof from about 0.03% to about 0.05% by weight based on the dry solids. 8.A coated floor underlayment prepared to receive an adhesive comprising:an uncoated floor underlayment comprising gypsum; and a coating appliedto the surface of said underlayment comprising: a film-forming latexpolymer; a surface enhancing agent selected to both accelerate thesetting reactions of calcined gypsum and to cross-link the adhesive; anda defoamer.
 9. The coated floor underlayment of claim 8 wherein saiduncoated floor underlayment comprises at least 50% gypsum on a dry,aggregate-free solids basis.
 10. The coated floor underlayment of claim8 wherein said uncoated floor underlayment comprises up to 30% cement ona dry, aggregate-free solids basis.
 11. The coated floor underlayment ofclaim 8 wherein said surface enhancing agent is present in said coatingin amounts of from about 50% to about 90% on a dry, aggregate-freesolids basis.
 12. The coated floor underlayment of claim 8 wherein saiduncoated floor underlayment comprises aggregate particles.
 13. A coatingfor preparing a floor underlayment to receive an adhesive comprising:from about 1 part to about 8 parts by volume water; and about 1 part byvolume of a coating mixture comprising: from about 10% to about 50% of aredispersible polymer based on the dry solids of said coating mixture;from about 50% to about 90% of a surface enhancing agent based on thedry weight of said coating mixture, said surface enhancing agent beingselected to function as both a set accelerator for calcined gypsum andalso as a cross-linking agent for the adhesive; and from about 0.15% toabout 0.25% of a defoamer based on the dry weight of said coatingmixture.
 14. A multi-layer floor underlayment comprising: a matrix layercomprising aggregate particles embedded in a crystalline matrixincluding calcium sulfate dihydrate; and a surface layer comprising alatex polymer, sulfate salts comprising a metallic cation and calciumsulfate hemihydrate distributed through an interlocking matrix ofcalcium dihydrate crystals.
 15. The underlayment of claim 14 whereinsaid sulfate salt comprises zinc sulfate.
 16. A method for strengtheningthe surface of a gypsum-containing floor underlayment comprising:providing a set and dried floor underlayment comprising gypsum;selecting a redispersible polymer; selecting a surface enhancing agentthat both accelerates setting reactions of calcium sulfate hemihydrateand cross-links a subsequently applied adhesive; blending theredispersible polymer; the surface enhancing agent and a defoamer toform a mixture; combining the mixture with water in proportions of fromabout 1:1 to about 1:8 by volume to form a coating; mixing said coating;applying the coating to the surface of the floor underlayment; anddrying the coating.
 17. The method of claim 16 wherein said mixing stepfurther comprises: stirring the coating from about 1 to about 5 minutes;waiting from about 10 to about 20 minutes; and stirring the coating fromabout 1 to about 5 minutes.
 18. The method of claim 16 wherein saidapplying step consists of one of spraying the coating, rolling thecoating, applying the coating with a mop and applying the coating with abrush.
 19. The method of claim 16 wherein said drying step furthercomprises allowing the coating to air dry for at least 2 hours.
 20. Themethod of claim 16 wherein said coating is stirred just prior toapplication.